In a cellular system, generally a mobile station terminal device searches for a cell to connect a wireless link. The cell is searched for using a synchronization channel (SCH) included in the radio frame of a downlink. Sometimes, in addition to the synchronization channel, a particular cell specific pilot channel or a broadcast channel (BCH) is used (see Non-patent document 1).
The first prior art described in Non-patent document 2 is explained with reference to FIGS. 1A, 1B and 2.
In this prior art, a plurality of SCH symbols is transmitted in a radio frame. On each SCH symbol, a generalized chip like series code (a GCL series code) is multiplexed in a frequency direction.
FIGS. 1A and 1B illustrate the multiplexing of SCH.
In FIG. 1A, frequency and time directions are taken on vertical and horizontal axes, respectively, and a radio resource is expressed. Furthermore, FIG. 1A illustrates how SCH is transmitted using the radio resource. SCH is located in a prescribed position in the time direction. S0, S1, S2, S3, . . . and SN−1 indicate each symbol of the GCL series code. Each symbol of the GCL series code is transmitted at the transmission timing of SCH, using one sub-carrier. When the number of sub-carriers is N, the GCL series code becomes a code of length N, composed of symbols S0 through SN−1.
The series number of the GCL series code multiplexed on each SCH symbol changes in the time direction. The pattern of change is a pattern having good cross-correlation and auto-correlation characteristic (called hopping code pattern in Non-patent document 2) and indicates an identifier for identifying a cell (or a cell group) and radio frame timing. Specifically, if the time change pattern of the series number of an SCH symbol transmitted from the cell of an identifier g for identifying a cell or cell group is as follows (Nsync: number of SCH symbols in a radio frame),
[Mathematical Expression 1]h(g)=(h0(g),h1(g),h2(g), . . . ,hNsync(g)−1)the GCL series code multiplexed on the i-th SCH symbol in a radio frame can be expressed as follows.
                    [                  Mathematical          ⁢                                          ⁢          expression          ⁢                                          ⁢          2                ]                                                                                  s                          h              i                              (                g                )                                              ⁡                      (            k            )                          =                  exp          ⁡                      (                                          -                j2π                            ⁢                                                          ⁢                              h                i                                  (                  g                  )                                            ⁢                                                k                  ⁡                                      (                                          k                      +                      1                                        )                                                                    2                  ⁢                                      N                    G                                                                        )                                              (        1        )            In the above expression, NG and k are the series length of a GCL series code and the number of a symbol, respectively. In the case k=0, it indicates the first (0-th) symbol of this GCL series code. Similarly, k=1, . . . and k=n indicate the first symbol, . . . and the n-th symbol, respectively.
FIG. 1B illustrates the case where four SCHs are time-multiplexed on one radio frame taking a frequency and time on the vertical and horizontal axes. In FIG. 1B, the GCL series code of an identifier g is multiplexed as an SCH. h(g)i is a hopping code pattern (an index number) used when generating the GCL series code of the identifier g. In FIG. 1B, four GCL series codes which have the same identifier specified by the same cell or cell group and the series number of the hopping code pattern of which are different are time-multiplexed.
FIG. 2 illustrated an example of a hopping code pattern.
It is the identifier g that indicates in what ordered row of this table the hopping code pattern is. For example, when the identifier g is 0, {4, 5, 6, 7 and 8} are listed as a hopping code pattern. In this case, the series length of the hopping code pattern is 5. Therefore, in the above example, as each series number, h(0)0=4, h(0)1=5, h(0)2=6, h(0)3=7 and h(0)4=8. Therefore, the hopping code pattern illustrated in FIG. 2 can be used when five SCHs are time-multiplexed on one radio frame.
On the receiving side, FFT is applied to the SCH symbols on the basis of the result of symbol and sub-frame timing detection performed before the detection process of the identifier of a cell (or a cell group) to transform the SCH symbols into a frequency domain. A sub-carrier component on which a GCL series code is multiplexed is extracted from the signal in the frequency domain and IDFT is applied to its differentially demodulated series code. The differential demodulation means to calculate S(n)×S*(n+1)=exp{j2πh(g)i(n+1)/NG} assuming the symbol of the n-th code as S(n). Thus, a value obtained by the differential demodulation becomes one obtained by rotating 2πh(g)i/NG integer times. Therefore, knowing how many times it is rotated, h(g)i can be known assuming that NG is already known. In reality, this is applied to all the SCH symbols in the radio frame and its IDFT output is stored in memory. Then in order to determine a hopping code pattern by applying soft-decision to it, the metric of all the circular shift patterns of a candidate hopping code pattern is calculated and the hopping code pattern of circular shift that obtains a maximum value is specified as the detection value of the identifier and radio frame timing of a cell (or a cell group). The metric calculation means to add the IDFT output values of S(n)×S*(n+1) obtained by the differential demodulation of all the hopping code patterns and all the circular patterns and to determine the largest added value to be a hopping code pattern to be obtained. For example, in the above example, IDFT output values obtained from the 0-th through fourth radio frames are stored as the function of n. Then, as to the IDFT output value obtained from the 0-th SCH, a value obtained when n=4 is taken. Similarly, as to those of the first, second, third and fourth SCHs, values obtained when n=5, n=6, n=7 and n=8, respectively, are taken and the value are added and stored. Then, a hopping code pattern given to n is circularly shifted and similarly a new added value are obtained and stored. Then, the same calculation is also applied to the hopping code patterns of other identifiers and their added values are stored. Then, after all the added values of the hopping code patterns of all the identifiers are obtained, the maximum value of them is searched for and the identifier and amount of circular shift of the hopping code pattern that gives this maximum value are obtained.
Another prior art is described in Non-patent document 3. The second prior art of Non-patent document 3 is explained with reference to FIGS. 3A and 3B.
In this prior art, a plurality of SCH symbols is transmitted in a radio frame. An orthogonal code for indicating a cell group identifier and radio frame timing (for example, a Walsh code) is multiplexed in the frequency direction. Different from the earlier-described prior art, a series number change pattern in the time direction does not indicate the cell group identifier and radio frame timing, but the series number itself indicates the cell group identifier and radio frame timing (and other information).
In order to increase the number of codes of a secondary SCH (the second synchronization channel), a method for multiplexing the plurality of orthogonal codes in the frequency direction is disclosed. FIG. 3A illustrates that a Walsh code is multiplexed in the frequency direction of an SCH. In this case, each symbol Wi (i=0 through N−1) is assigned to each sub-carrier and the code length is N. According to the nature of a Walsh code, there are only N types of Walsh codes of length N. therefore, As illustrated in FIG. 3B, the identifier multiplexes Walsh codes g and f of length M in the frequency direction. In this case, it is assumed that 2M=N. Then, the number of codes that can be used for an SCH the number of whose sub-carriers is N becomes M×M since a Walsh code of length M and a Walsh code of length M are combined. For example, if M=4 and N=8, in the case of FIG. 3A, the number of usable Walsh codes is 8 (N=8), while in the case of 3B, it becomes 16 (M×M=16) and it increases.
On the receiving side, the FFT of SCH symbols are performed on the result of symbol and sub-frame timing detection performed before the detection process of the identifier of a cell group, the correlation process of SCHs is performed in the frequency domain and cell group identifier and radio framing timing is detected.
Patent document 1 discloses a technology for shifting the transmitting phase of each down frame transmitting for each TCH and transmitting it in order to improve the accuracy of channel estimation.    Non-patent document 1: 3GPP TR25.814 V7.0.0    Non-patent document 2: 3GPP TSG-RAN WG1, R1-061117, “Comparison of One-SCH and Two-SCH schemes for EUTRA CELL Search”, ETRI    Non-patent document 3: 3GPP TSC-RAN WG1, R1-060780, “SCH Structure and Cell Search Method foe E-UTRA Downlink”, NTT DoCoMo, NEC    Patent document 1: Japanese Patent Laid-open Publication No. H10-126331
In the above prior arts, since the series number of a series code multiplexed on each SCH in a radio frame differs, it is necessary to perform IDFT, etc. and a correlation process, using codes of all the series numbers for each receiving SCH at the time of SCH detection on the receiving side and thereby the amount of process increases.